1. Field of the Invention
This invention relates generally to water monitoring systems used in remote, unattended locations employing detectors whose operation is adversely affected by biofouling. Most commonly, the detectors are electrochemical sensors such as dissolved oxygen sensors. More particularly, the invention involves a protection system wherein electrochemical sensors, small electrodes (e.g., coated wires), chemical microsensors, optical probes, and the like in a submersed water monitoring device are exposed to a gaseous antibiofoulant in the intervals between sequential measurement operations. The anti-fouling gas atmosphere inhibits microbial colonization on the sensor surfaces thereby increasing the useful life of the sensors.
Chemical microsensors are broadly defined as small, chemically sensitive systems whose components can be microfabricated. An example of such a system is a micro gas chromatograph which employs a spiral capillary column lithographically defined on a glass microscope slide.
2. Description of the Related Art
In the past, biological fouling organisms in natural waters have prevented the measurement of dissolved oxygen (DO) where frequent tending and cleaning of the sensors is prevented by the remoteness of the measurement location.
The primary limitation of long-deployment dissolved oxygen measurement systems is the tendency of their exposed DO sensors to accumulate biotic fouling. The growth of microorganisms or even macroorganisms commonly degrades the performance of electrochemical dissolved oxygen sensors to the extent that they cannot provide reliable signals.
Dissolved oxygen concentration is a variable of natural water environments which is directly related to the establishment of aerobic/anerobic chemical processes and life processes of all animal life. The productivity of fisheries, for example, has been found to be a function of the dissolved oxygen concentration of the water. Localized conditions of hypoxia (dissolved oxygen concentrations less than 2 milligrams per liter) or anoxia (DO below 100 micrograms per liter) have been implicated in mortalities and migration of commercially important species in bottom waters of the northern continental shelf near Louisiana and Texas. Anoxic conditions along the New Jersey coast during the summer of 1976 resulted in a $60-million loss to the shellfish industry.
Knowledge of the dynamics of dissolved oxygen in natural waters is critical for effective management of water and water-borne resources. At present, an adequate base of time-correlated data on dissolved oxygen, temperature, and salinity is neither available nor obtainable with existing technology. A long-deployment instrument system containing oxygen probes together with temperature and salinity sensors would provide the necessary data for researchers seeking to understand and forecast hypoxic and anoxic events. Without such data, an effective management program for this economically important resource cannot be developed.
In natural waters, conditions of reduced dissolved oxygen are usually established over periods of days, and the concentration will vary irregularly. Monitoring of dissolved oxygen concentration provides the most significant information for analysis purposes when monitoring is performed over time periods which are similar in length to the periods of dissolved oxygen variation. Such time periods can be lengthy, ranging up to a month or more. Because.of biofouling, unattended dissolved oxygen and other electrochemical sensors deployed in natural waters usually become fouled with microorganisms before a suitable number of observations can be collected. Thus, what is needed is a data logging system capable of unattended operation without degradation of performance due to biofouling of its sensor elements.
Methods available in the past to prevent fouling of sensor devices have employed the use of toxic materials adhered as a coating on the surface of the sensor. The anti-fouling coating has afforded some protection for a limited period of time, but has also caused a reduction in the sensitivity of the sensor and its general utility. [Edgerton, U.S. Pat. No. 4,092,858 (1978), col. 1, lines 28-34] The present invention, by applying a gas phase antibiofoulant in the time intervals between the measurement phases of operation, avoids the problems associated with toxic antifouling coatings--deterioration over time and reduced sensitivity of the sensor.
Some sewage treatment plants are reported to employ automatic devices which clean dissolved oxygen sensors by periodically withdrawing them from the treatment tanks and directing jets of detergent solution onto their surfaces. Such a treatment system is in part mechanical and in part chemical, but it is clearly impractical for a submersed, remote monitoring system.
Measurement of dissolved oxygen in remote ponds has been done over extended periods by systems utilizing solar powered mechanical lifting devices. The dissolved oxygen sensors are periodically lifted out of the water so as to kill biofoulants by desiccation. This procedure, however, is impractical for monitoring systems which remain submerged.
At least two United States patents address the problem of biofouling on water monitoring devices. Grana et al., U.S. Pat. No. 4,089,209 (1978), "Remote Water Monitoring System," describes an apparatus which collects samples, performs sample analyses with electrochemical sensors, and electronically transmits data to, and receives command signals from a remote station. The solenoid-actuated valves which admit the water samples are said to "operate to eliminate cavities where water could collect and stagnate, and when [opened], operate to remove marine growth attached to the surface of the sampling unit from the path of inflowing water." [col. 2, lines 47-50] The control module of the device is able to store data for subsequent transmission via telemetry upon receipt of an interrogation signal. [col. 3, lines 17-24] The patent addresses the problem of marine organism growth on the surface of the sample unit [col. 5, lines 27-32], but makes no provision for cleaning its electrochemical sensors. Since the sensors remain dry until a water sample is admitted and since sensor readings are taken shortly after sampling, biofouling of the sensors is not likely a problem.
Edgerton, U.S. Pat. No. 4,092,858 (1978), "Oceanographic Sensor with In-Situ Cleaning and Bio-Fouling Prevention System," describes a piezoelectric crystal with sensing elements deposited on, or attached to, the inner and outer surfaces of the transducer or sensing core. [col. 2, lines 36-42] "When excited electrically, at or near resonance, the sensor core vibrates such that all fouling matter is removed from the surface of the sensor and also such excitation prevents any growth to occur." [col. 1, lines 64-67] Cleaning is performed by using greater energy intensity levels than that required for biofouling prevention. [col. 2, lines 10-12] It is said that this system for cleaning or biofouling prevention can be used with any undersea device that can be driven or vibrated in-situ continuously or intermittently by a piezoelectric transducer to cause acoustic streaming and/or cavitation forces. [col. 4, lines 29-34]
Edgerton is addressed to a problem generally similar to that of the present invention, but it relies on ultrasonic vibration to remove or prevent biofouling on the surface of its sensor elements. The sensing elements must be deposited on or attached to the surfaces of an ultrasonic transducer. Edgerton refers to sensing elements fabricated from platinum or gold, or from detector materials such as piezo-resistance materials or other semiconductor materials. [col. 2, lines 36-41] Edgerton makes no mention of dissolved oxygen sensors; nor does he suggest that dissolved oxygen sensors or reference electrodes (e.g., standard calomel electrodes) could be attached to his transducers and function properly. In any case, the present invention follows an approach very different from that of Edgerton.